1. Field of the Invention
The present invention relates to a back-illuminated photodetector such as a back-illuminated charge coupled device (CCD) or back-illuminated active pixel sensor (APS) for detecting a radiation having a large absorption coefficient such as ultraviolet ray, .gamma.-ray, or charged particle beam, and a method of fabricating the same.
2. Related Background Art
As a photodetector for detecting the energy distribution of an electromagnetic wave or light containing charged particles as an image, a CCD is known. The full frame transfer (FFT) CCD is mainly used for measurement purposes.
A general CCD based on the FFT has no storage portions, and individual imaging portions can have a large size. Because of the large photosensitive area, this device prefers for weak light level. However, incident light must reach the imaging portion through a polysilicon electrode or protective PSG film formed on the surface of the device. Since such an element formed on the surface absorbs input light having a large absorption coefficient, e.g., light with a wavelength of 400 nm or less, the light does not reach the imaging portion, so high sensitivity cannot be expected for light in such a short wavelength region.
To detect light with a short wavelength, a back-illuminated CCD can be used as it has an imaging portion with a substrate as thin as about 10 to 30 .mu.m and illuminates from the opposite side of the electrode formation surface. In this back-illuminated CCD, even light having a short wavelength reaches the imaging portion without being largely absorbed, so high sensitivity can be obtained even in a short wavelength band of about 200 nm. The device can also be applied to, e.g., X-ray imaging for medical use.
In the X-ray imaging, an image is difficult to reduce and project on the imaging portion of the CCD, unlike the normal visible light image sensing, and a CCD having an imaging surface with almost the same size as that of the object must be prepared. A CCD to be used for chest X-ray radiography must be a large CCD or a buttable CCDS. However, the back-illuminated CCD with a thin imaging portion is fragile and is difficult to increase its size. For the firm CCD, a thick frame is fitted on the imaging portion. However, even a device of this type can hardly have a large size, and the frame around the imaging portion is dead space.
Techniques for solving this problem are disclosed in Japanese Patent Laid-Open Nos. 53-114361 (to be referred to as "prior art 1" hereinafter), 62-30373 (to be referred to as "prior art 2" hereinafter), 6-268183 (to be referred to as "prior art 3" hereinafter), and 6-291291 (to be referred to as "prior art 4" hereinafter).
In prior art 1, a CCD is bonded on a support substrate via boro-silicate glass (BSG) or boro-phospho-silicate glass (BPSG) softened by heating, and then, a thin imaging portion is formed. This prior art is characterized in that V-shaped grooves are formed in the bonding surfaces of the CCD and the support substrate in advance.
In prior art 2, boro-silicate glass is deposited thick on that surface of a CCD or a substrate having a CCD, which is opposite to the illuminated surface, and is fired to form a reinforcing member.
In prior art 3, a sapphire substrate is anodically bonded on the illuminated surface of a CCD. In prior art 4, a protective insulating film is formed on the device surface side with a CCD and planarized, and then, a glass substrate is anodically bonded on the protective insulating film.